Serious investments in improving the energy efficiency of urban transportation systems would have a huge payback for cities around the world to the tune of $70 trillion in savings, says the International Energy Agency (IEA).

The savings comes from money that no longer has to be spent on vehicles, fuel and infrastructure.

The need for cities to act is urgent, IEA says, because more than half the world's population lives in cities, which are already suffering from traffic jams that translate into billions of dollars in lost fuel and countless hours of productivity - aside from the negative impact they also have on environmental quality, health and safety.

"As the share of the world's population living in cities grows to nearly 70% by 2050 and energy consumption for transport in cities is expected to double, the need for efficient, affordable, safe and high-capacity transport solutions will become more acute," says IEA Executive Director Maria van der Hoeven.

"Urgent steps to improve the efficiency of urban transport systems are needed not only for energy security reasons, but also to mitigate the numerous negative climate, noise, air pollution, congestion and economic impacts of rising urban transport volumes."

Policy makers must take a long-term view to address the challenges, continues van der Hoeven. "Governments must think beyond individual technologies and electoral cycles, and consider how to build - and how to renew - cities that will accommodate and transport nearly 6.3 billion people by 2050," she adds. "We must plan infrastructure, logistics and energy systems now that make sense today and over the coming decades."

IEA's report, "A Tale of Renewed Cities," gives examples from more than 30 cities across the globe that illustrate the way forward.

New York saved 11 minutes off travel times after introducing express bus service and when Seoul reformed its bus system - reversing policies that encouraged crowding - both ridership and safety improved across the city. After revamping its urban rail system, Belgrade saw passenger levels triple in the first six months.

Based on their experiences, IEA recommends policies in three broad categories:

"Avoid" policies that allow people to avoid travel altogether or avoid increasing transport-related energy consumption, such as incentives for tele-commuting and better logistics technologies.

"Shift" to policies that encourage citizens and commuters to start using more energy-efficient modes of transportation, such as public transit, walking, cycling or freight rail.

"Improve" policies that lead to greater efficiency, such as more stringent fuel-economy standards and use of cleaner vehicles, including hybrids or plug-in electric cars or buses.

If these are fully implemented across the transportation sector, cities would save that $70 trillion because that's how much less they would need to spend on oil, roads and other infrastructure and vehicles.

Forget one-size-fits all approaches

Cities will require different policies, depending on their state of evolution, says IEA:

Developing – Urban areas with low densities, an inadequate travel infrastructure, and weak public transit options have a "rare opportunity" to embrace policies that discourage or penalize sprawling development and prioritize denser cores, among other things.

Sprawling – Regions with weak urban cores, and commercial and business hubs spread throughout a wide area. One way to move to more efficient transportation systems in these situations is to increase density, but it can take years of planning and development to reverse existing policies or behavior. IEA advocates policies that help make public transportation options more appealing than private alternatives.

Congested – Cities with heavy road traffic, often tied to peak travel hours. Here, it's important to adopt strategies that discourage vehicle ownership to help reduce or stabilize vehicle volume; and to invest in improved traffic management technologies, such as real-time travel information.